static <A> Producer<A> moveProducerToNewThread(Producer<A> producer, CloseablesHolder ch, int bufferSize default 65536) {
  if (producer == null) null;
  SimpleCircularBuffer<A> buffer = new(bufferSize);
  new Flag done;
  new Var<Throwable> error;
  ch.add(() -> done.raise());
  startThread(r {
    try {
      A a;
      while ping (!done.isUp() && (a = producer.next()) != null) {
        while (buffer.isFull() && !done.isUp()) sleep(1);
        buffer.add(a);
      }
    } catch e {
      error.set(e);
    } finally {
      done.raise();
    }
  });
  
  ret () -> {
    while true {
      A a = buffer.popFirst();
      if (a != null) ret a;
      if (done.isUp())
        null;
      else
        sleep(1);
    }
  };
}